This invention relates to systems and methods for mixing cement slurries at oil or gas well sites and for pumping such slurries into the wells.
After completing the drilling of an oil or gas well, a cement slurry is typically pumped into the well to isolate the pay zone and provide support for pipe in the well. Important parameters for the cement slurry are density and pumping rate.
Cement density is important for two reasons. First, the density defines the ratio of dry cement powder to water which determines the properties of the slurry and the hydrated cement. These properties include friction pressure, setting time, cement strength, etc. Second, density also maintains proper well control through hydrostatic head of the cement column. The hydrostatic head prevents the pressurized fluids in the reservoir from producing uncontrollably into the well.
Friction pressure is also a factor of pumping rate. A high friction pressure can fracture the formation, thereby allowing the cement to flow out into the reservoir. Also, pumping time is determined by pumping rate. The slurry must be placed in the well within a specified time to prevent the cement from hardening in the drill string.
Another aspect of cementing an oil or gas well is that typically more than one type of cement slurry needs to be prepared at the well site and pumped into the well. This is done sequentially with one slurry being mixed and pumped into the well and then the next being mixed and pumped into the well, pushing the previous slurry or slurries farther into the well. Different slurries that have different densities and different compositions require different control parameters. The total volume of each such slurry needs to be tracked to ensure placement of the respective slurries at desired locations in the well.
Prior systems and methods have provided automatic control of cement density but have not combined this feature with automatic pumping control. These prior systems and methods also have not provided for pre-entering multiple sets of cement mixing and pumping control parameters in such a manner that permits either manual or automatic switching from one set to another for sequentially mixing and pumping different cement slurries into the well. Such a system and method for overcoming these shortcomings is needed to provide improved control of the sequential mixing and pumping of multiple types of cement slurries into an oil or gas well.